General: El Niño episodes (left hand column) reflect periods of exceptionally warm sea surface temperatures across the eastern tropical Pacific. La Niña episodes (right hand column) represent periods of below-average sea-surface temperatures across the eastern tropical Pacific. These episodes typically last approximately 9-12 months. Sea-surface temperature (top) and departure (bottom) maps for December - February during strong El Niño and La Niña episodes are shown above.

Detailed: During a strong El Niño ocean temperatures can average 2°C – 3.5°C (4°F - 6°F) above normal between the date line and the west coast of South America (bottom left map). These areas of exceptionally warm waters coincide with the regions of above-average tropical rainfall.  During La Niña temperatures average 1°C - 3°C (2°F - 6°F) below normal between the date line and the west coast of South America. This large region of below-average temperatures coincides with the area of well below-average tropical rainfall.

For both El Niño and La Niña the tropical rainfall, wind, and air pressure patterns over the equatorial Pacific Ocean are most strongly linked to the underlying sea-surface temperatures, and vice versa, during December-April. During this period the El Niño and La Niña conditions are typically strongest, and have the strongest impacts on U.S. weather patterns.

El Nino and La Niña episodes typically last approximately 9-12 months. They often begin to form during June-August, reach peak strength during December-April, and then decay during May-July of the next year. However, some prolonged episodes have lasted 2 years and even as long as 3-4 years. While their periodicity can be quite irregular, El Niño and La Niña occurs every 3-5 years on average.